A telephone test set is useful for performing various functions during routine maintenance operations on a telephone line by a telephone line technician. When the telephone line technician is communicating with the central office, it often happens that the call is place on hold. An auxiliary speaker was provided in an arrangement disclosed in U.S. Pat. No. 4,691,336, assigned to the assignee of the present invention. The speaker permitted the telephone line technician to place the unit on the technician's belt and listen to communications coming down the line from the central office which are amplified and broadcast by the loudspeaker while permitting the technician to perform other functions. The technician could use the amplifier and loudspeaker to listen for voice synthesized dispatches from the central office and communications from automated test equipment located in the central office.
Although allowing a technician to monitor a line without holding the test set to the ear, the test set disclosed in U.S. Pat. No. 4,691,336 is not really "hands-free" for two-way conversation, because the transmitting portion of the test set is muted when the amplified speaker is on to prevent acoustic feedback. Thus, the operator cannot talk back to the caller at the other end.
A microprocessor controlled "test set" incorporating a digital multimeter and a speakerphone has been produced by Ziad, Inc. A disadvantage of this test set is that it does not provide what is known as "dry loop operation". Dry loop operation is the use of a pair of test sets connected across opposite ends of a dead telephone pair, enabling the linepersons to identify a particular pair. In dry loop operation, the pair-under-test does not receive battery voltage from the Central Office. Instead a small external battery of 3 volts or less is usually used in series to provide the minimum voltage necessary to bias the polarity protection bridge in the test set. Dry loop operation precludes the use of electronic, integrated loop interface and speech network circuits for basic test set (non-speakerphone) use because of the minimum voltage and current required. An all passive speech network comprising a 2-to-4 wire hybrid transformer, carbon transmitter, and dynamic receiver must be used instead. The architecture of a speakerphone is incompatible with such a passive network.
There is a need for a test set which permits dry loop operation and still provides for hands-free two-way communication via a speakerphone.